A coordinated series of immuno- and histochemical experiments are proposed to study the intrinsic organization of the gustatory recipient zones of the nucleus of the solitary tract (NST) and parabrachial complex (PBC) in the hamster. These brainstem relay nuclei are treated as a functional pair and are directly compared. Comparisons are also made between the hamster and rat. Special emphasis is placed on the association of morphologically distinct classes of neurons and putative neurotransmitters. Previous work in the gustatory NST are compared to the new findings on the PBC. The distributions of tyrosine hydroxylase (TH)-, substance P-,and methionine enkephalin-immunoreactivity and NADPH-diaphorase activity are examined throughout the PBC. New studies on the distribution of calbindin D-28 (CaBP)-immunoreactivity in the gustatory brainstem relay nuclei are also proposed.The known association of dopamine and NADPH-diaphorase in other sensory systems is tested in the gustatory NST using double-labeling methods as specific cell markers. These experiments are designed to identify classes of gustatory neurons with distinguishing chemical signatures but not necessarily unique morphological features, and to establish features that serve to characterize the gustatory brainstem relay nuclei. Th addition, the termination patterns of chorda tympani fibers onto gustatory neurons in the NST are explored with a combination of anterograde degeneration and immunohistochemical techniques. The chorda tympani is destroyed with the toxic lectin ricin and the same tissue is processed for immunohistochemistry using TH and CaBP anti-sera. The distribution of degenerating terminal boutons onto immunoreactive dendrites is characterized. These studies are designed to identify classes of neurons receiving direct projections from taste receptors. This proposal extends our analysis of the gustatory system to the next relay in the ascending pathway and continues our efforts to clarify the basic anatomical principles governing the processing of gustatory information.